This invention relates generally to a protection circuit for a switched mode power supply (SMPS) and specifically to a low cost, fast acting over-current protection circuit for such supplies.
Switched mode power supplies are extensively used because of their high efficiency in supplying varying output load current requirements. They conventionally include a developed source of DC voltage supplying a pair of transistor switches that control current flow in the primary of a power transformer. The secondaries of the power transformer are in turn coupled to rectifier networks for developing different DC output voltages. The transistors are switched or driven by a pulse width modulator (PWM) that, in turn, is coupled to an output of the SMPS for changing the duty cycle of the switching transistors in accordance with load current requirements.
Many such power supplies use a current sensing transformer in series with the switching transistors and the primary of the power transformer for sensing overloads in the primary winding current. The sensing transformer develops an AC voltage in its secondary winding that is proportional to current flow in its primary winding. This current is the power transformer primary current which is also the input current to the SMPS. The sensed output voltage is rectified to produce an indicating voltage which is applied to one input of a voltage comparator, the other input of which is supplied with a fixed reference voltage. The output of the voltage comparator is coupled to a latch circuit that is effective for shutting down the PWM when the comparator senses that excessive input current is flowing in the power transformer primary winding.
Switched mode power supplies are relatively slow in changing the duty cycle of the switching transistors in response to output load current changes. The slow response is generally desirable because a rapid response to output current changes could easily result in undesirable "hunting." The slow response time of the SMPS does, however, give rise to difficulty in the event of a large over-current or a short circuit condition existing at a high current capacity voltage output.
The reference voltage is normally established at a value that is higher than the indicating voltage developed by the sensing circuit under the highest peak input current encountered during normal operating conditions. This point usually occurs during load step changes with the supply operating at normal levels. For example, the load current may change from a low value to a high value and result in peak input currents that are many times greater than the currents existing under steady state maximum power output conditions. An SMPS with a maximum power output of 200 watts would experience peak currents of about 4 amperes in its power transformer primary winding. During normal load step changes, the primary current could peak at 8 amperes. For such a supply, the over current trip point would therefore be set at more than 8 amperes. Ten amperes is a reasonable shutdown or trip level for such a supply. When the SMPS output circuits are lightly loaded (operation with minimum load), a short circuit across a low voltage, high current capacity output will result in most of the output power being dissipated in the short circuit. For a 5 volt high capacity output and a 10 ampere primary current shutdown level, input power would rise to 850 watts or more with current flow into the short circuit reaching 100 amperes before shutdown occurs. Such high currents exert great stress on the SMPS system components and could be very destructive to the load that caused the short circuit.
With the protective circuit of the invention, the switched mode power supply functions normally with respect to input shutdown current capability, but will shut down at a very much lower input current level under a short circuit condition. The benefits of the invention are achieved with a few additional low cost parts and only minor changes in circuitry.